[OOPs] ext4: fixpoint divide exception at ext4_fill_super+0x141c/0x2908

[OOPs] ext4: fixpoint divide exception at ext4_fill_super+0x141c/0x2908

[Nageswara R Sastry]

Hi,

Found kernel bug - fixpoint divide exception
While working with fsfuzzer

Environment: 2.6.31-rc7
Architecture: s390 and ppc64

------------[ cut here ]------------ 

Kernel BUG at 000003e00429d934 [verbose debug info unavailable] 

fixpoint divide exception: 0009 [#1] SMP 

Modules linked in: ext4 jbd2 crc16 loop autofs4 lockd sunrpc ipv6 
qeth_l2 qeth
q
dio vmur ccwgroup dm_round_robin dm_multipath scsi_dh sd_mod scsi_mod 
multipath
dm_snapshot dm_zero dm_mirror dm_region_hash dm_log dm_mod dasd_fba_mod
dasd_eck
d_mod dasd_mod ext3 jbd 

CPU: 2 Not tainted 2.6.31-rc6 #1 

Process mount (pid: 2675, task: 000000003ed06038, ksp: 0000000036217920) 

Krnl PSW : 0704200180000000 000003e00429d934 (ext4_fill_super+0x1478/0x2908
[ext
4]) 

            R:0 T:1 IO:1 EX:1 Key:0 M:1 W:0 P:0 AS:0 CC:2 PM:0 EA:3 

Krnl GPRS: 0000000000000100 0000000000000100 0000000000000006 
000000002fc61800
            0000000000000000 0000000000000100 000000002fc61800 
000000002fc61800
            000000003143a800 00000000000000e0 0000000000000000 
000000003eb0d800
            000003e00427c000 000003e0042b8958 000003e00429d87c 
0000000036217b10
Krnl Code: 000003e00429d928: b9160050           llgfr   %r5,%r0 

            000003e00429d92c: a7490000           lghi    %r4,0 

            000003e00429d930: b987004a           dlgr    %r4,%r10 

           >000003e00429d934: b91600a5           llgfr   %r10,%r5 

            000003e00429d938: a53f80d0           llill   %r3,32976 

            000003e00429d93c: a7ad000c           mghi    %r10,12 

            000003e00429d940: b904002a           lgr     %r2,%r10
            000003e00429d944: 1895               lr      %r9,%r5
Call Trace:
([<000003e00429d87c>] ext4_fill_super+0x13c0/0x2908 [ext4])
  [<00000000000f46f2>] get_sb_bdev+0x13e/0x19c
  [<000003e00429230e>] ext4_get_sb+0x2e/0x40 [ext4]
  [<00000000000f3f98>] vfs_kern_mount+0xc0/0x168
  [<00000000000f40ac>] do_kern_mount+0x58/0x114
  [<000000000010e558>] do_mount+0x798/0x830
  [<000000000010e6a0>] SyS_mount+0xb0/0x100
  [<00000000000266be>] sysc_noemu+0x10/0x16
  [<0000004e53f234e2>] 0x4e53f234e2
Last Breaking-Event-Address:
  [<000003e00429d8d8>] ext4_fill_super+0x141c/0x2908 [ext4]

---[ end trace b5563edf9c0c9b52 ]---

*P.S. If you need any information please let me know. Please cc me as I 
am not subscribed to the list.

Thanks and Regards
R.Nageswara Sastry

Re: [OOPs] ext4: fixpoint divide exception at ext4_fill_super+0x141c/0x2908

[Nageswara R Sastry]

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Hi,

The following bug exists in 2.6.32-rc* also.

 > ------------[ cut here ]------------
 > Kernel BUG at 000003e00429d934 [verbose debug info unavailable]
 > fixpoint divide exception: 0009 [#1] SMP

Please find the patch which solves the following 'fixpoint divide 
exception'. I tested the same and not seeing any KERNEL BUG/exception.

I created the patch from 2.6.32-rc6 code.


Thanks and Regards
R.Nageswara Sastry

Though the 'sbi->s_log_groups_per_flex' contains valid data after the 
shift operation 'groups_per_flex' is becoming zero, which later used as 
a divisor.
groups_per_flex = 1 << sbi->s_log_groups_per_flex

Signed-off-by: Nageswara R Sastry <rnsastry@linux.vnet.ibm.com>




[-- Attachment #2: ext4_fix_point_division_error.patch --]
[-- Type: text/plain, Size: 813 bytes --]

diff -Naurp a/linux-2.6.32-rc6/fs/ext4/super.c b/linux-2.6.32-rc6/fs/ext4/super.c
--- a/linux-2.6.32-rc6/fs/ext4/super.c	2009-11-03 20:37:49.000000000 +0100
+++ b/linux-2.6.32-rc6/fs/ext4/super.c	2009-11-04 13:21:48.000000000 +0100
@@ -1681,6 +1681,12 @@ static int ext4_fill_flex_info(struct su
 	sbi->s_log_groups_per_flex = sbi->s_es->s_log_groups_per_flex;
 	groups_per_flex = 1 << sbi->s_log_groups_per_flex;
 
+	/* There are some situations, after shift the value of 'groups_per_flex'
+	   can become zero and division with 0 will result in fixpoint divide exception
+	 */
+	if (groups_per_flex == 0)
+		return 1;
+
 	/* We allocate both existing and potentially added groups */
 	flex_group_count = ((sbi->s_groups_count + groups_per_flex - 1) +
 			((le16_to_cpu(sbi->s_es->s_reserved_gdt_blocks) + 1) <<

Re: [OOPs] ext4: fixpoint divide exception at ext4_fill_super+0x141c/0x2908

[Nageswara R Sastry]

[-- Attachment #1: Type: text/plain, Size: 526 bytes --]

Seems to be I have a little format related issue with the patch. So 
resending it. Please accept my apologies.
Sachin thanks for letting me know.

>  > ------------[ cut here ]------------
>  > Kernel BUG at 000003e00429d934 [verbose debug info unavailable]
>  > fixpoint divide exception: 0009 [#1] SMP
> 
> Please find the patch which solves the following 'fixpoint divide 
> exception'. I tested the same and not seeing any KERNEL BUG/exception.
> 
> I created the patch from 2.6.32-rc6 code.

Regards
R.Nageswara Sastry



[-- Attachment #2: ext4_fix_point_division_error.patch --]
[-- Type: text/plain, Size: 1087 bytes --]

Though the 'sbi->s_log_groups_per_flex' contains valid data after the shift operation
'groups_per_flex' is becoming zero, which later used as a divisor.
groups_per_flex = 1 << sbi->s_log_groups_per_flex

Signed-off-by: Nageswara R Sastry <rnsastry@linux.vnet.ibm.com> 
---

diff -Naurp a/linux-2.6.32-rc6/fs/ext4/super.c b/linux-2.6.32-rc6/fs/ext4/super.c
--- a/linux-2.6.32-rc6/fs/ext4/super.c	2009-11-03 20:37:49.000000000 +0100
+++ b/linux-2.6.32-rc6/fs/ext4/super.c	2009-11-04 13:21:48.000000000 +0100
@@ -1681,6 +1681,12 @@ static int ext4_fill_flex_info(struct su
 	sbi->s_log_groups_per_flex = sbi->s_es->s_log_groups_per_flex;
 	groups_per_flex = 1 << sbi->s_log_groups_per_flex;
 
+	/* There are some situations, after shift the value of 'groups_per_flex'
+	   can become zero and division with 0 will result in fixpoint divide exception
+	 */
+	if (groups_per_flex == 0)
+		return 1;
+
 	/* We allocate both existing and potentially added groups */
 	flex_group_count = ((sbi->s_groups_count + groups_per_flex - 1) +
 			((le16_to_cpu(sbi->s_es->s_reserved_gdt_blocks) + 1) <<

Re: [OOPs] ext4: fixpoint divide exception at ext4_fill_super+0x141c/0x2908

[Theodore Tso]

On Wed, Nov 04, 2009 at 07:24:04PM +0530, Nageswara R Sastry wrote:
> Seems to be I have a little format related issue with the patch. So  
> resending it. Please accept my apologies.
> Sachin thanks for letting me know.
>
>>  > ------------[ cut here ]------------
>>  > Kernel BUG at 000003e00429d934 [verbose debug info unavailable]
>>  > fixpoint divide exception: 0009 [#1] SMP
>>
>> Please find the patch which solves the following 'fixpoint divide  
>> exception'. I tested the same and not seeing any KERNEL BUG/exception.

Nageswara,

Thanks for sending this patch.  The problem with it is that it leaves
sbi->s_log_groups_per_flex non-zero, but it leaves sbi->s_flex_groups
unallocated.  This should lead to number of kernel oops caused by a
null pointer dereference if there is any attempt to allocate blocks or
inodes, or to resize the filesystem.

A better fix is would be:

ext4: Avoid divide by zero when trying to mount a corrupted file system

If s_log_groups_per_flex is greater than 31, then groups_per_flex will
will overflow and cause a divide by zero error.  This can cause kernel
BUG if such a file system is mounted.

Thanks to Nageswara R Sastry for analyzing the failure and providing
an initial patch.

http://bugzilla.kernel.org/show_bug.cgi?id=14287

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
diff --git a/fs/ext4/super.c b/fs/ext4/super.c
index d4ca92a..8662b2e 100644
--- a/fs/ext4/super.c
+++ b/fs/ext4/super.c
@@ -1673,14 +1673,14 @@ static int ext4_fill_flex_info(struct super_block *sb)
 	size_t size;
 	int i;
 
-	if (!sbi->s_es->s_log_groups_per_flex) {
+	sbi->s_log_groups_per_flex = sbi->s_es->s_log_groups_per_flex;
+	groups_per_flex = 1 << sbi->s_log_groups_per_flex;
+
+	if (groups_per_flex < 2) {
 		sbi->s_log_groups_per_flex = 0;
 		return 1;
 	}
 
-	sbi->s_log_groups_per_flex = sbi->s_es->s_log_groups_per_flex;
-	groups_per_flex = 1 << sbi->s_log_groups_per_flex;